A known guide bar includes guide plates on respective longitudinal sides of the guide bar in the region of the guide bar tip. The guide plates laterally extend beyond the guide paths formed by a guide groove in the guide bar. The radial projection of the guide plates with respect to the guide groove is so provided that the chain links of a saw chain are substantially laterally covered and only the cutting links of the saw chain extend beyond the edge of the guide plates. The saw chain is guided in the guide groove of the guide bar. In this way, it is ensured that the penetration depth of the cutting links into the material to be cut is reduced during plunge cutting work. Difficulties with respect to the manipulation during plunge cutting work are intended to be reduced thereby.
Too large a projection of the guide plates leads, however, to a reduction of the cutting power and plunge cutting work during tree maintenance is thereby hindered. On the other hand, if the projection of the guide plates (especially in the direction-changing region of the guide bar) is too low, then the manipulation of the motor-driven chain saw is made more difficult thereby.
It is an object of the invention to provide a guide bar wherein the guidance of the saw chain is improved while maintaining a good cutting power during plunge cutting work and the guide bar can be used for saw chains of different configuration.
The guide bar of the invention is for a portable handheld work apparatus including a motor-driven chain saw or overhead branch cutter, the guide bar being for a saw chain including cutting links, lateral connecting links and center drive links pivotally connected to each other via rivets or pins with the drive links having respective rakers formed thereon. The guide bar includes: an elongated planar base body defining an outer periphery wherein a peripheral guide groove is provided for receiving the rakers therein and for guiding the saw chain along the outer periphery; the guide bar having guiding surfaces along the outer periphery for supporting the lateral connecting links and the cutting links and the guiding surfaces being separated from each other by the guide groove; the guide bar having a direction-changing section at the outer free end thereof for guiding the saw chain over the outer free end of the guide bar; guide plates mounted on the base body in the direction-changing section and extending radially beyond the guiding surfaces to laterally cover the connecting links and the cutting links of the saw chain; an idler sprocket rotatably mounted in the direction-changing section; the guide plates being configured as rotating left and right guide discs and the guide discs being connected to the idler sprocket so as to rotate therewith; and, the left guide disc having a set of radially projecting first sections and the right guide disc having a set of radially projecting second sections arranged offset with respect to corresponding ones of the first sections.
According to the invention, the guide plates in the direction-changing section (at the tip of the guide bar) are configured as rotating guide discs and are connected to the idler sprocket so as to rotate therewith. In this way, a reliable guidance of the guide bar on a motor-driven chain saw or an overhead branch cutter is combined with good cutting power with the aid of the guide plates on the guide bar. In lieu of a circularly-shaped edge, the guide discs include sections which project radially and each cutting link of the saw chain comes partially into overlapment with a radially projecting section. The radially projecting sections can have an outline which is rectangular, rhombic, trapezoidal or define a polygon in another manner.
The left and right guide plates on the direction-changing section of the guide bar alternately overlap a cutting link because of the arrangement of the radially projecting sections offset in the running direction. The cutting links each have a front cutting edge running essentially radially and this cutting edge has a contour laterally offset to the outside referred to the running direction of the saw chain and therefore projects axially beyond the outer surfaces of the guide discs in the region of the cutouts between the radially extending sections. Referred to the running direction of the saw chain, a depth limiter is positioned ahead of the front cutting edge. It can be advantageous to so arrange the radially projecting sections that they axially overlap the depth limiter. Alternatively to this, it is, however, also possible to so place the saw chain that the depth limiters are disposed in the region of a radial edge of the radially projecting section of the opposite-lying guide disc. The radial edge runs ahead when viewed in the peripheral direction. With the above-mentioned constructive measures, the function of the depth limiter as an element, which limits the penetration depth of the particular cutting link, is supported by the arrangement of one of the radially projecting sections to the depth limiter and the manipulation of the saw chain is simplified. On the other hand, the cutting power of each front cutting edge is not hindered by the radially projecting sections and therefore a good cutting power especially in plunge cutting operations is ensured.
Difficulties with respect to the manipulation of the guide bar during plunge cutting work do not occur when the radially projecting sections (measured in the peripheral direction of the guide discs) are spaced so far on a particular guide disc that two cutting links partially and one connecting link can come to lie in the cutout between each two radially projecting cutouts. It is practical to so arrange the edges of the radially projecting sections that the edges are angled so as to depart from the perpendicular. Perpendiculars are here understood to be center perpendiculars of the guide discs and parallels to the center perpendiculars of the guide discs. It is seen as practical to configure the cutouts between the radially projecting sections of each guide disc in the peripheral direction approximately as long as the width of the radially projecting sections. To make possible uniform plunge cutting work with the guide bar, it is practical to end the edge of the guide discs with a slight spacing radially below a roof cutting edge of the cutting link.
The idler sprocket is surrounded on both sides by side plates in the direction-changing section. These side plates radially project beyond the idler sprocket and axially guide the latter. In each side plate, a bearing is provided for rotatably journalling the idler sprocket and the guide discs which run with the idler sprocket. The guide discs are preferably connected to the idler sprocket so as to rotate therewith and are driven by the movable saw chain. Preferably, the guide discs are fixed at the direction-changing section of the guide bar so as to be exchangeable and can be exchanged with the saw chain.
It is practical to provide each guide disc with three or even two radially projecting sections. To be able to use conventional saw chains in combination with the guide bar, it is practical to configure the idler sprocket with a number of teeth which amounts to four times the number of radially projecting sections. In an advantageous embodiment, the idler sprocket has eight or twelve teeth. It can be practical to so configure the guide bar that it can be turned over. For this purpose, the guide discs are configured in their basic outline to be axially symmetrical.